The present invention relates to hydraulic chain tensioners for internal combustion engines.
The invention is particularly concerned with a hydraulic chain tensioner comprising: a support body forming a cylindrical housing receiving a piston slidable in the housing and carrying a pressure pad acted upon by the piston and arranged to come into contact with the chain, a high-pressure chamber defined by the housing and by the piston and arranged for communication with a pressurized hydraulic liquid thereby to supply the tensioner with lubricating liquid and to complete the filling of the high-pressure chamber.
Hydraulic tensioners of this type are already known for chains or other transmission elements referred to as endless linkages such as those described for example in FR-A-2 610 685. These tensioners are connected to a source of pressurized hydraulic liquid such as the oil pump of the heat engine. The tensioner exerts pressure via its piston onto the chain. When the chain becomes slack, the piston exits its housing so that the pressure of the liquid in the housing falls and when this pressure is less than the pressure of the hydraulic liquid, the valve opens and hydraulic liquid enters the so-called high-pressure chamber of the tensioner. The tensioner opposes the vibrating movements of the chain since its chamber contains a liquid which is incompressible in principle.
However, sometimes the hydraulic liquid, i.e. the oil of the lubrication circuit, contains dissolved gasses or gasses in the form of an emulsion. These gasses can separate progressively from the liquid and form a cushion of air in the high-pressure chamber of the tensioner. The behaviour of the tensioner is then changed in relationship to the volume of compressed gas. Moreover, and in particular after a prolonged stoppage of the engine, it may be the case that the tensioner does not immediately carry out its function after the engine has been started up, for as long as the pressure of the hydraulic liquid supplied by the liquid source fails to reach the appropriate level.
The present invention proposes to develop an improved hydraulic tensioner for an endless chain or linkage of the type first referred to above.
To this end the invention relates to a hydraulic tensioner of the type initially defined above having the high-pressure chamber connected to the ambient surroundings via a gas-purging orifice.
According to one advantageous preferred feature, a compression spring acts between the support body and the pad pressing on the endless chain or linkage. This spring makes it possible to absorb vibrations of the chain when the engine is first started up, when the high-pressure chamber has not yet been charged with pressurized liquid.
This spring may be helicoidal compression spring surrounding the piston and bearing against an edge of the housing receiving the piston.
The spring can alternatively be a helicoidal compression spring housed in the high-pressure chamber between the base of said chamber and the adjacent end of the piston.
In both cases it is advantageous to provide the device with a locking member permitting the pad to be held in a retracted piston for mounting operations and arranged to be released only when the tensioner has been installed.